This application is related to U.S. Pat. Application Ser. No. 603,704 entitled "In Situ Recovery of Shale Oil", filed Aug. 11, 1975 by Gordon B. French , now U.S. Pat. No. 4,043,595 to U.S. Pat. Application Ser. No. 603,705 entitled "Forming Shale Oil Recovery Retort Into Slot-Shaped Columnar Void", filed Aug. 11, 1975 by Richard D. Ridley, now U.S. Pat. No. 4,043,596, and to U.S. Pat. Application Ser. No. 790,350, entitled "In Situ Oil Shale Retort With a Horizontal Sill Pillar", filed Apr. 25, 1977, by Ned M. Hutchins. All three of these applications are assigned to the assignee of the present application and are incorporated herein by this reference.
This invention relates to recovery of liquid and gaseous products from oil shale. The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which upon heating decomposes to produce hydrocarbon liquid and gaseous products. The formation containing kerogen is called "oil shale" herein, and the hydrocarbon liquid product is called "shale oil".
One technique for recovering shale oil includes forming an in situ oil shale retort in a subterranean formation containing oil shale. At least a portion of the formation within the boundaries of the in situ oil shale retort is explosively expanded to form a fragmented permeable mass of particles containing oil shale. The fragmented mass is ignited near the top of the retort to establish a combustion zone. An oxygen-containing gas is introduced into the top of the retort to sustain the combustion zone and cause it to move downwardly through the fragmented permeable mass of particles in the retort. As burning proceeds, the heat of combustion is transferred to the fragmented mass of particles below the combustion zone to release shale oil and gaseous products therefrom in a retorting and vaporization zone. Vaporized constituuents of shale oil, water vapor and the like may condense on cooler oil shale in the retort below the retorting zone. The retorting zone moves from top to bottom of the retort ahead of the combustion zone, and the resulting shale oil and gaseous products pass to the bottom of the retort for collection and removal. Recovery of liquid and gaseous products from oil shale deposits is described in greater detail in U.S. Pat. No. 3,661,423, to Donald E. Garrett, assigned to the assignee of this application.
In preparing for the retorting process the formation containing oil shale should be fragmented rather than simply fractured to create good and uniform permeability so that undue pressures are not required to pass the gas through the retort, and so that valuable deposits of oil shale are not bypassed owing to non-uniform permeability. The aforementioned patent applications disclose techniques for fragmenting a substantial volume of formation in a retort site to form a fragmented mass of particles in an in situ oil shale retort. The in situ retort is formed by excavating a void in the retort site, drilling blasting holes into the remaining portion of the formation in the retort site, loading explosive into the blasting holes, and detonating the explosive to expand the formation toward the void.
To promote uniformity of particle size and permeability of the fragmented mass, and to minimize the quantity of explosives, the blasting holes should be reasonably accurately located with respect to each other, and with respect to the void toward which expansion occurs during the explosion. Oil shale formations in the western United States are often between 50 to about 500 feet thick or even more, and are covered by a non-productive overburden, which may be thousands of feet deep, thus often making it difficult to drill from the surface and accurately locate blasting holes in the oil shale formation.
In one embodiment disclosed in application Ser. No. 790,350, and entitled "In Situ Oil Shale Retort With A Horizontal Sill Pillar", an open base of operation is excavated in the formation at a working level near the top of an in situ retort to be formed, which may be a thousand feet, or more, below the ground surface. A substantially horizontal access drift is excavated at a production level below the base of operation to provide access to a lower portion of the retort site. A void is excavated above the access drift so the void opens into the access drift and terminates below the base of operation at the top of the fragmented mass being formed. This leaves a substantially horizontal portion of intact formation between the top of the void and the bottom of the base of operation. The blasting holes for explosive for expanding formation are drilled from the base of operation through the formation on opposite sides of the void. Inasmuch as the working level is much closer to the top of the retort being formed than the distance from the retort to the overburden at the ground surface, this permits more accurate and rapid drilling of blasting holes from the base of operation than from the ground surface. This, in turn, facilitates explosive expansion to form the fragmented mass of oil shale particles in the retort.
In an embodiment disclosed in application Ser. No. 790,350, a horizontal sill pillar of unfragmented formation remains between the top of the fragmented mass in the retort and the bottom of the base of operation. The sill pillar has a number of bore holes through it after formation of the fragmented mass. Such bore holes include the upper ends of blasting holes drilled from the base of operation. Such bore holes can be used for access from the base of operation for establishing and sustaining a combustion zone in the fragmented mass below the sill pillar.
In the past a variety of techniques have been developed for igniting oil shale particles in an in situ retort in order to establish a combustion zone. One such technique is disclosed in U.S. Pat. application Ser. No. 578,203, filed May 16, 1975, and now U.S. Pat. No. 4,027,917, and U.S. Pat. No. 3,952,801, issued Apr. 27, 1976, to Robert S. Burton, III, and assigned to the assignee of this application. According to the techniques disclosed in these patents, a hole is bored to the top of the fragmented permeable mass and a burner is lowered through the bore hole to the oil shale to be ignited. A mixture of a combustible fuel such as LPG (liquefied petroleum gas) and oxygen containing gas such as air, is burned in the burner, and the resultant flame is directed downwardly toward the fragmented permeable mass. The burning is conducted until a substantial portion portion of the oil shale has been heated above its ignition temperature so that combustion of the oil shale in the fragmented mass is self-sustaining. After ignition an oxygen supplying gas is introduced to the retort to advance the combustion zone through the fragmented mass.
It can be time consuming to establish a combustion zone in an oil shale retort. For example, a startup time as long as a week has been experienced with a retort in the South/Southwest portion of the Piceance Creek structural basin in Colorado. Such a long startup time results in consumption of large quantities of LPG, an expensive, premium fuel.
An in situ oil shale retort can have a substantial lateral extent. For example, it can be square with a lateral dimension of 120 feet or more. In such a large retort, a large number of burners and bore holes to various portions of the top of the retort and large quantities of fuel such as LPG can be required for establishing a combustion zone in the retort. Preparation of a large number of bore holes and use of a large number of burners and large quantities of LPG can contribute significantly to the cost of producing hydrocarbon products from oil shale.
If a combustion zone is not properly established across the top of the retort, the resulting combustion zone can be skewed and/or warped. It is desired to establish and maintain a combustion zone which is flat and uniformly transverse to the direction of its advancement to maximize yield of hydrocarbon products from the oil shale in an in situ oil shale retort. If the combustion zone is skewed relative to its direction of advancement, there is more tendency for oxygen present in the combustion zone to migrate into the retorting zone, thereby oxidizing hydrocarbon products produced in the retorting zone and reducing hydrocarbon yield. In addition, with a skewed and/or warped combustion zone, excessive cracking of hydrocarbon products produced in the retorting zone can result.
Around each locus of ignition in the fragmented mass, a combustion zone is formed which tends to advance downwardly and laterally in the fragmented permeable mass. The combustion zone advances downwardly through the fragmented mass primarily by gas flowing through the fragmented mass, and it advances laterally and radially in the fragmented mass primarily by conduction and radiation. Inasmuch as heat transfer by conduction and radiation through a fragmented mass is slower than heat transfer by gas flow, a substantial amount of unretorted oil shale can be left in the "corners" or outer regions adjacent the boundaries of the fragmented mass. This can significantly reduce the yield of hydrocarbons obtained from the retort.
Thus, it is desirable to provide a method for establishing in a combustion zone in an in situ oil shale retort which effectively reduces startup time, as well as labor, fuel, energy and/or equipment costs. Moreover, it is desirable to provide a method which effectively initiates combustion so that the resulting combustion zone will be essentially flat and transverse to its direction of advancement, without leaving substantial amounts of unretorted oil shale in the top of the fragmented mass.